1. Field of the Invention
The present invention generally relates to color toner used for electrophotography, and more particularly, to color toner that forms clear images by having a uniform conductivity and a good electrostatic charge characteristic.
2. Description of the Related Art
Electrophotography is a technique that is widely used for image forming apparatuses such as photocopiers and facsimiles. In the electrophotography technique, photoconductive insulator is usually used as described in the U.S. Pat. No. 2,297,691, for example.
An electrostatic latent image is formed by applying a photo signal provided by lasers or LEDs to the face of the photoconductive insulator that is charged by corona discharge or a charging roller.
Then, resin power called toner is adhered on the electrostatic latent images to develop a visible toner image. In the development of the toner image, the toner is charged by frictional electrification with a magnetic carrier or a blade.
The toner image is transferred to recording media such as paper or film. The toner image is, however, just xe2x80x9cputxe2x80x9d on the recording media. The fixing of the toner image is necessary.
As the last step, the toner on the recording media is melted by applying heat, pressure, or light, and solidified to fix the toner image on the recording media.
As described above, toner containing thermoplastic resin as the principal component is fixed on recording media by being melted and then solidified on the recording media.
Two methods are used to melt the toner: a heating roller method in which the recording media is directly heated and pressed by a heating roller and a flash fixing method in which flash light is applied to the recording media.
Recent demand for color printing requires toner that is colored by pigment and/or dye.
In the case of black toner, the toner is colored by carbon having high conductivity. Since the black toner has higher conductivity (lower resistance), the development of black toner images is easy and they are produced with little edge effect. Additionally, the black toner is not excessively charged even if it is charged repeatedly.
Compared to the black toner, the color toner does not contain carbon. Because the color toner has lower conductivity (higher resistance), the development of color toner images is rather difficult.
To eliminate this problem, conductive powder is adhered on the surface of the particle of the color toner according to a conventional technique.
Since the conductive powder, however, is not fixed on the surface of the particle of the color toner, the conductive powder easily falls off when the color toner is mixed with the magnetic carrier, for example. Consequently, the resistance of the color toner rises so that the characteristics of the developing powder changes resulting in unstable developments.
To eliminate this problem, a technique in which the conductive powder is distributed in the binding resin of the color toner so that the conductive powder does not fall off is proposed.
The problem here is that the conductive powder of more than 20 wt % must be contained in the color toner to make the resistance of the color toner low enough. Even if whitish conductive powder is generally used to avoid its effect on the color of the color toner, the color toner becomes whitish and causes the saturation of the fixed image of the recording media.
The resistance of the color toner becomes low enough if metal powder having a conductivity of less than 1 xcexa9xc2x7cm is used as the conductive powder distributed in the binding resin even at the weight percentage of less than 20 wt %. In this case, however, the resistance of the color toner becomes too low to charge the color toner, especially when particles of the metal powder are exposed on the surface of the particles of the color toner.
A color printer capable of printing a plurality of color toner images each corresponding to a different color must adjust development conditions such as a development bias voltage if the resistance of one color toner differs from that of another color toner.
The adjustment of the development conditions makes the control circuit of the color printer complex. The development conditions may change if color toner images are developed repeatedly. Development of color toner having uniform resistance regardless of its color is desired.
Accordingly, it is a general object of the present invention to provide a novel and useful color toner by which one or more of the problems described above are eliminated.
Another and more specific object of the present invention is to provide color toner having appropriate resistance, even if a limited weight percentage of conductive powder is contained, that can produce a color image of clear tone.
To achieve one of the above objects, a color toner for electrophotography according to the present invention, includes, binding resin, colorant, and conductive particles having an electrical resistance of 1-100 xcexa9xc2x7cm, an aspect ratio of equal to or more than 10, and a major axis length of equal to or less than 4 xcexcm that are distributed in the binding resin at a weight percentage of equal to or less than 20 wt %.
According to the present invention, the color toner contains stick-shaped conductive particles having an aspect ratio of 10 or more. Compared with the related art, the conductive particles easily contact with each other and effectively reduce the resistance of the color toner even if only a small quantity of conductive particles is contained in the color toner. Accordingly, the weight percentage of the conductive particles can be reduced to 20 wt % or less, which results in a clear printed image.
The greater the aspect ratio of the conductive particles is, the less conductive particles are required in order to reduce the electrical resistance of the color toner. If the conductive particles are too long, however, the conductive particles may be exposed on the surface of a color toner particle and the color toner becomes too easily discharged. The major axis length is desired to be 4 xcexcm or less. Furthermore, the electrical resistance of the conductive particles is desired to be 1-100 xcexa9xc2x7cm to ensure that the conductive particles of 20 wt % or less suffice the requirement but the resistance of the color toner is high enough.
The color toner according to the present invention as described above, is characterized in that the main component of the conductive particles is one of ZnO, TiO2, SnO2, Al2O3, In2O3, SiO2, MgO, BaO, MoO3, WO, and MoO3.
Those metal oxides have a desirable resistance and less effect on the color of the color toner because of their own light colors or because they are colorless. Accordingly, clear fixed images are obtained by the development of toner images.
The color toner according to the present invention as described above, is further characterized in that the conductive particles are made of TiO2 or SnO2, and 50 wt % or more of the conductive particles have a major axis of equal to or more than 1 xcexcm long and a minor axis of equal to or less than 0.1 xcexcm.
Stick-shaped titanium oxide and/or tin oxide are, even in small quantity, are especially effective in reducing the electrical resistance of the color toner. When the conductive components contain particles having a major axis of 1 xcexcm long or more and a minor axis of 0.1 xcexcm long or less at a content ratio of 50 wt % or more of the total conductive particles, the effect is obvious. Accordingly, the color toner containing these conductive particles forms clear color images because of the good properties of the color toner.
The color toner according to the present invention as described above is yet further characterized in that the TiO2 is coated by a coating layer made of SnO2 or Sb2O3, or both.
The conductive coating layer formed on the TiO2 conductive particle further reduces the required quantity of the conductive particles contained in the color toner keeping the quality of the color image at the same level.
Other objects, features, and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawing.